Although the invention is applicable to sprockets for use with various types of chains, the invention will be described with reference to a sprocket used with a roller chain. A conventional sprocket generally has a tooth form defined in ISO (International Organization for Standardization) 606:1994(E), the tooth form being hereinafter referred to as “ISO tooth form.” The ISO tooth form is depicted in FIG. 6, and FIG. 7 shows a roller chain engaging a sprocket having the ISO tooth form.
The parameters of the ISO tooth form, as shown in FIG. 6, are defined in ISO 606:1994(E) as follows:d=p/sin(180°/z)df=d−d1re(max)=0.12d1(Z+2)ri(min)=0.505d1α(max)=140°−90°/zre(min)=0.008d1(Z2+180)ri(max)=0.505d1+0.069d11/3α(min)=120°−90°/zβ=360°/zwherein                p is the chain pitch (chord pitch),        d is the diameter of pitch circle,        d1 is the outer diameter of the chain roller,        ri is the radius of the arc of the tooth gap bottom,        re is the radius of the tooth surface,        df is the root diameter, i.e., the diameter of an inscribed circle tangent to the tooth gap bottom,        z is the number of sprocket teeth, and        α is the roller seating angle, i.e., the angle of the tooth bottom arc.β is the tooth pitch angle as shown in FIG. 6, pc is the pitch line of the sprocket, and ha is the height of a tooth head, as measured from the pitch polygon.        
As apparent from the above expressions, in the ISO tooth form shown in FIG. 6, the tooth gap bottom 7 is in the form of an arc having a radius ri, which is slightly larger than the radius (d1/2) of chain roller 9. The front and back tooth surfaces 3 and 4, which are both continuous with an arcuate tooth gap bottom 7, are each in the form of an arc of radius re. The diameter df of the tooth gap bottom circle is equal to the difference between diameter d of the pitch circle pc and the diameter d1 of the roller. The diameter df of the tooth gap bottom circle is also substantially equal to the difference between the diameter d of the pitch circle pc and twice the radius ri of the tooth bottom circle.
As is generally known, when a roller chain is engaged with spaced sprockets to form a transmission, and the driving sprocket has an ISO tooth form, the chain undergoes polygonal movement upon engagement with the driving sprocket, and the polygonal movement results in chordal vibration.
As shown in FIG. 7, a roller 9a of chain 8 engages a tooth gap 6 to contact a tooth gap bottom 7 at an engagement portion. However, the adjacent following roller 9b does not come into contact with a back tooth surface 4 before it reaches the engagement portion. L1 designates the track of travel of the chain as it approaches the engagement portion. Track L1 is at its lower limit in the left side view in FIG. 7. When the sprocket 1 rotates by a half the pitch angle, that is, through an angle β/2, the tooth gap bottom 7, with which roller 9a is engaged, is raised, and the track L2 is correspondingly raised, through a distance δ, to an upper limit as depicted on the right side view in FIG. 7. Thus, as the tooth gap bottom 7 moves up and down in FIG. 7, the chain 8 undergoes polygonal movement corresponding to the rotation of the pitch polygon shown in FIG. 6, and the approaching part of the chain is moved up and down as a result of the polygonal movement.
As a result, even if the sprocket 1 rotates at a constant speed, polygonal movement causes the chain to pulsate, and the pulsating movement causes chordal vibration, which generates noise. If the cycle of the polygonal movement resonates with the natural vibration of the approaching chain span, a chordal vibration of larger magnitude is generated. Polygonal movement also results in intermittent changes in the speed of the chain 8. As shown in FIG. 7, when a sprocket having the ISO tooth form is rotated, the contact position 10 between the tooth gap bottom 7 and the chain roller 9 does not move relative to the sprocket.
A sprocket composed of teeth having an asymmetrica tooth form has been proposed in order to suppress chordal vibration due to a polygonal movement of a roller chain. In the proposed sprocket, the front tooth surfaces (that is, the front surfaces with reference to the rotational direction of the sprocket) are asymmetric in relation to the back tooth surfaces. In a transmission utilizing such a sprocket, polygonal movement of the chain is suppressed by first bringing a roller of the chain into contact with a front tooth surface at an engagement location. Such a sprocket is described in U.S. Pat. No. 5,921,878, dated Jul. 13, 1999.
In a roller chain transmission in which the driving sprocket has an ISO tooth form, since the roller engages a tooth gap bottom directly, without coming into contact with either a front tooth surface or a back tooth surface, impact noise is also generated.
Polygonal movement, resulting chordal movement, vibration noise, intermittent changes in speed, and impact noise, occur not only in transmissions utilizing roller chains, but also in chain transmissions in which the chains have no rollers, and instead, pins or bushings come into direct contact with the sprocket. Additionally, although the transmission in U.S. Pat. No. 5,921,878 is intended to address the problems caused by polygonal movement, there is room for improvement in the sprocket disclosed in that patent.
Accordingly, objects of this invention are to solve the above-mentioned problems encountered in prior art chain transmissions, and to provide a sprocket, and a chain transmission device using the sprocket, which suppress polygonal movement of the chain, reduce noise, and prevent intermittent changes in speed.